Although the most commercially used semiconductor today is silicon, there are other semiconductors, such as GaAs and InP, that are presently of commercial and technological interest. Unlike silicon which, except for minor amounts of elements which are typically present to determine the conductivity type and unwanted impurities, is an elemental semiconductor, these other semiconductors are often alloys. That is, they have two or more major atomic constituents present as well as minor amounts of dopants and impurities. In addition to the GaAs and InP previously mentioned, other alloy semiconductors which are presently of possible commercial interest are GeSi, AlGaAs, AlInAs, and InGaAsP. The subscripts typically used to indicate the precise composition have been omitted for reasons of conciseness.
The semiconductor alloys are typically grown epitaxially on substrates or other epitaxial layers which may have the same or a different composition. For example, AlGaAs and GeSi may be grown on GaAs and Si, respectively. If the compositions differ, the lattice constants almost invariably differ and a strain is present in the epitaxial layers.
In these semiconductor alloys, the isoelectronic constituent atomic species occupy random sites in the lattice. Isoelectronic means that the atomic species have the same valence electrons. Such a structure may be viewed as being disordered as there is no long range correlation between the positions of a particular isoelectric species. However, ordered alloys are known and ordered semiconductor alloys have been observed. For example, U.S. Pat. No. 4,205,329 issued on May 27, 1980 to Dingle, Gossard, Petroff and Wiegmann (Gossard et al) describes a superlattice comprising a periodic structure of (GaAs).sub.n (AlAs).sub.m where n and m are the numbers of contiguous monolayers of GaAs and AlAs, respectively.
Transitions between a disordered structure and an ordered structure are relatively common and well studied in metallic alloys. The appropriate annealing of such systems at temperatures lower than a critical temperature induces the different constituent atoms to occupy particular lattice sites and to thereby produce long-range order in the resulting structure. The transition can be one of two types. First, it can be isostructural and the lattice type is not changed by the transition. Secondly, it can be neostructural and the lattice type is changed by the transition. The transitions may be driven by various physical mechanisms including differences in electronegativity, valence electron number or size between the ordering atoms. The thermodynamics of the transition is discussed in, for example, Thermodynamics of Solids, Richard A. Swalin, John Wiley and Sons, pp. 153-159. The particular order-disorder transition discussed by Swalin includes the transition present in Cu-Au alloys.
However, such order-disorder transitions are not known in semiconductors. For example, bulk GeSi is a model random alloy. In spite of experiments in which prolonged annealing has been performed on bulk GeSi at various temperatures, such annealing has been unable to induce the different atomic species to occupy specific lattice sites and the alloy appears to obey ideal solution theory. See, for example, Constitution of Binary Alloys, McGraw-Hill Book Company, 1958, pp. 774-775. Such transitions were not described by Gossard et al.